MRI apparatus with high-resistance magnet

Abstract

Eddy current generated around a magnetic circuit in an MRI apparatus is one of the causes of deviation from an ideal magnetic field gradient waveform and causes image distortion, loss of strength, ghost generation, loss of signal, and spectral distortion. An object of the present invention is to suppress the generation of the eddy current. In an MRI apparatus, a ferromagnetic material formed from powder is used in a part of a magnetic circuit: the powder mainly comprising a mother phase containing iron or cobalt and showing ferromagnetism; and a high-resistance layer having a resistance not less than ten times as high as the mother phase and a Vickers hardness lower than that of the mother phase being formed in layers along parts of the surface of the powder on parts or the entire of the surface.

1. An MRI apparatus comprising:
 a ferromagnetic material formed from powder; and 
 a magnetic circuit using said ferromagnetic in a part thereof; 
 wherein said powder to be mainly a mother phase of said ferromagnetic material contains iron or cobalt with ferromagnetism; and 
 said powder is provided with a high-resistance layer which has a resistance higher than or equal to ten times as high as said mother phase and a Vickers hardness lower than that of said mother phase, and that is formed in layer structure along the surface of said powder on parts or the entire of said surface. 
 
   
   
     2. An MRI apparatus comprising:
 a ferromagnetic material formed from powder; and 
 a magnetic circuit using said ferromagnetic in a part thereof; 
 wherein said powder to be mainly a mother phase of said ferromagnetic material contains iron or cobalt with ferromagnetism; and 
 said powder is provided with a high-resistance layer which has a resistance higher than or equal to ten times as high as said mother phase and a Vickers hardness lower than that of said mother phase at room temperature, and that is formed in layer structure with the thickness of 10 to 10,000 nm along the surface of said powder on parts or the entire of said surface. 
 
   
   
     3. An MRI apparatus comprising:
 a static magnetic field generator for forming a static magnetic field having a uniform magnetic field strength distribution in a measuring space; and 
 a ferromagnetic material for adjusting the uniformity of said magnetic field in said measuring space; 
 wherein said ferromagnetic is comprised of an iron or cobalt type material characterized by a high resistance. 
 
   
   
     4. An MRI apparatus comprising:
 a static magnetic field generator for forming a static magnetic field having a uniform magnetic field strength distribution in a measuring space; 
 a ferromagnetic material for adjusting the uniformity of said magnetic field in said measuring space; and 
 a pair of permanent magnets disposed on opposite sides of said measuring space in said static magnetic field generator; 
 wherein said permanent magnets are comprised of an iron or cobalt type material characterized by a high resistance. 
 
   
   
     5. An MRI apparatus comprising:
 a static magnetic field generator for forming a static magnetic field having a uniform magnetic field strength distribution in a measuring space; 
 a ferromagnetic material for adjusting the uniformity of said magnetic field in said measuring space; and 
 a pair of permanent magnets disposed on opposite sides of said measuring space in said static magnetic field generator so as to face said measuring space of magnetic poles in said static magnetic field generator; 
 wherein said permanent magnets are comprised of an iron or cobalt type material characterized by a high resistance. 
 
   
   
     6. An MRI apparatus comprising:
 a static magnetic field generator for forming a static magnetic field having a uniform magnetic field strength distribution in a measuring space; 
 a ferromagnetic material for adjusting the uniformity of said magnetic field in said measuring space; and 
 a pair of permanent magnets and high-frequency shields disposed on opposite sides of said measuring space in said static magnetic field generator so as to face said measuring space of magnetic poles in said static magnetic field generator; 
 wherein said permanent magnets or high-frequency shields are comprised of an iron or cobalt type material characterized by a high resistance. 
 
   
   
     7. An MRI apparatus comprising:
 a static magnetic field generator for forming a static magnetic field having a uniform magnetic field strength distribution in a measuring space; 
 a ferromagnetic material for adjusting the uniformity of said magnetic field in said measuring space; and 
 a pair of permanent magnets and high-frequency shields disposed on opposite sides of said measuring space in said static magnetic field generator so as to face said measuring space of magnetic poles in said static magnetic field generator; 
 wherein said permanent magnets or high-frequency shields are comprised of an iron or cobalt type material whose grain boundaries are provided with high resistance layers; 
 wherein said high-resistance layers comprise fluorine compounds. 
 
   
   
     8. An MRI apparatus comprising:
 a static magnetic field generator for forming a static magnetic field having a uniform magnetic field strength distribution in a measuring space; 
 a ferromagnetic material for adjusting the uniformity of said magnetic field in said measuring space; and 
 a pair of permanent magnets and high-frequency shields disposed on opposite sides of said measuring space in said static magnetic field generator so as to face said measuring space of magnetic poles in said static magnetic field generator; 
 wherein said permanent magnets or high-frequency shields are comprised of an iron or cobalt type material whose grain boundaries are provided with high resistance layers; 
 wherein said high-resistance layers comprise oxyfluorine compounds. 
 
   
   
     9. An MRI apparatus according to any one of  claims 1  to  8 , wherein said a magnetic material characterized in that a high-resistance layer is a fluorine compound containing fluorine and one or more kinds of an alkali metal, an alkali-earth metal, a transition metal, and a rare-earth metal is used.